Could your stronghold be at risk?

In the campaign I'm running, my players have put their money together and built their first stronghold. (They built it around a magic portal they found, in the hopes of establishing their own trade monopoly. My players are great.) However, I'm not sure how often their fledgling domain should suffer random encounters. The rules make it fairly clear how often parties camping in the outdoors should have random encounters, but it's much less clear how often a stronghold should risk random encounter rolls.

One random encounter roll each day seems a bit much - especially if the party is trying to spend a month doing downtime activities, or is away on a dungeon crawl. But on the other hand, saying that their stronghold doesn't suffer from random encounter rolls at all seems a bit strange, given that they built it out in the borderlands.

I'm hoping there's some paragraph of the rules I've missed that explains this, but if not, I need advice: How often should I be rolling random encounters for a stronghold my players own?

I dont think there is any paragraph explaining it (unless I have also missed it).

I typically have events of note as encounters which can be anything from a strange occurances on the horizon to some internal issues to some band of (roving) beastmen deciding the stronghold may be a good idea to raid.  Essentially things that the populous will talk about or be dealing with for a while after the event.

As to increment I have plans to making some chance of an encounter about once per month (not quite at the point of the group having a stronghold yet). I.E. A random encounter roll each month.

I'm not sure if the Core Rules cover this in any detail, but Axioms 3 has an article (Wandering Into War) discussing random encounters for domains:

If I recall the core rules correctly, any stronghold of sufficient value and garrison, built in a cleared hex, civilizes at minimum the hex it’s built in.

This would suggest a random encounter rate of one per month.

Of course, if they didn’t clear the hex, or if they don’t have a garrison, or if the value isn’t enough, well, then it’s still wilderness/borderlands, and they would have the random encounters of 1/week or 1/day as appropriate.

(I am not an Autarch but that’s my opinion.)

I agree with Aryxmaraki, as that's exactly my read of the rules. Building a Stronghold before you've cleared the hex is painful!